feat: table declarations

[aljazerzen]

Closes #2646 with adding support for:

    module my_schema {
      let my_table <[{ id = int, a = text }]>
    }

    let another_table <[{ id = int, b = text }]>

    my_schema.my_table
    join another_table (==id)
    take 10

The rule is that any variable declaration with relational type and without an assigned value will be treated as a table in the database.

Fully qualified name is used to refer to that table. This will be changed soon.

[max-sixty]

What's the utility of this? Relative to leaving them as they're specified in the query? (not claiming confidently that there isn't an advantage!)

[max-sixty]

or this will obviate the need for this?

Fully qualified name is used to refer to that table. This will be changed soon.

[aljazerzen]

This adds an ability to specify a module as a "database module":

module my_database {
    module my_schema {
        let my_table <[{}]>   # this will be `my_schema.my_table`
    }
}

module another_database {
    let another_table <[{}]>  # this will be `another_database`
}

The problem is that these two tables are defined in two different databases, so they cannot be present in the same query. That's why "current database" is a compiler parameter, so it knows for which database it is compiling for.

Idea being that in the future, we could support querying multiple databases, so PRQL source would compile into multiple queries that would be sent to each of the databases.